Currently, at least two belt layers containing steel cords arranged slantly at a certain cord angle with respect to an equatorial plane of a tire are mainly used in a belt of a radial tire for a passenger car such that the cords of these layers are crossed with each other. Also, a belt reinforcing layer comprising nylon cords or the like is disposed on an outside of the belt in a radial direction in order to ensure the stability during the running of the tire and particularly prevent the peeling of the belt layer in the high-speed running (particularly, peeling notably produced at an end portion of the belt layer) to improve the durability.
At the present day, it is common to adopt the belt reinforcing layer for improving the high-speed durability. As a material of the cord in the belt reinforcing layer is mainly used a nylon being low in the heat build-up and cost. In the radial tire having such a belt reinforcing layer, however, it is recently demanded to further reduce the road noise and flat spot and improve the pushing-out property at the end portion of the belt in the high-speed running.
As such a countermeasure, a high-elastic cord composed of an aromatic polyamide (aramid) or the like, which has an elasticity higher than that of the nylon and a high glass transition point (Tg) and is effective for the lowering of the road noise and flat spot, is applied to the belt reinforcing layer. However, the high-elastic cord has a problem in view of the cost because it is expensive.
Moreover, as a material having an elasticity higher than that of the nylon but being low in the cost is known a polyethylene terephthalate (PET). However, the polyethylene terephthalate is large in the lowering of the elastic modulus at higher temperatures, and has a problem that the sufficient effect of suppressing the pushing-out cannot be developed as the temperature of the tire elevates during the high-speed running. Since a filament composed of the polyethylene terephthalate does not have a functional group on its surface, the adhesiveness to rubber is remarkably inferior as compared with that of the nylon in the adhesive treatment with a usual RFL adhesive liquid. Therefore, the use of the polyethylene terephthalate in the belt reinforcing layer can lower the road noise and flat spot, but has a problem that the high-speed durability deteriorates notably.
On the contrary, in order to improve the high-speed durability of the tire wherein the cords made of the polyethylene terephthalate are used in the belt reinforcing layer, there are proposed many means for improving the adhesiveness between cord and rubber (JP-A-2001-9927, JP-A-2001-63312, JP-A-2001-98245 and JP-A-2000-248254).